Field of the Invention
The present disclosure relates to an optical scanning apparatus capable of emitting one or more laser beams and an image forming apparatus mounting the optical scanning apparatus.
Description of the Related Art
Image forming apparatus has realized high processing speed and enhanced image resolution. Along with this, the optical scanning apparatus is advanced to multi-beam scanning, in which two or more plurality laser beams are used. Also, electrophotography printer, capable of forming variety of screens used in the printing market into high quality image, is desired. However, in multi-beam scanning, when position gap in a main scanning direction is caused on an image on a photosensitive drum formed by each beam, the position gap is periodically caused in a sub-scanning direction. Due to this, moire is caused or image is distorted. To solve such problems, an image forming apparatus disclosed in WO2013/161259(A1) measures the phase difference between a plurality of laser light beams which determines a writing start position in the main scanning direction and adjusts light emitting timing of each laser beam based on the measured result.
Further, due to an increase in temperature in the apparatus due to heat generated during operation, reflective index of lens may vary in the optical scanning apparatus. In this case, the phase difference between the laser beams varies. The image forming apparatus disclosed in WO2013/161259 (A1) detects the variation amount of the phase difference between the laser beams using a beam detect (BD) sensor and corrects the variation in the phase difference caused by the increase in temperature according to the detection result. That is, the image forming apparatus measures the phase difference between a head laser beam and an end laser beam of a plurality of the laser beams received by the BD sensor, compares the measured data with data measured at the time of shipping from a factory, and detects the variation amount of the phase difference. According to the variation amount detected, the image forming apparatus corrects a writing start position (light emitting start timing) of each laser beam.
An example of the image forming apparatus is briefly explained as follows. FIG. 12 is a schematic diagram of the laser beams detected by the BD sensor. Following explains the contents of (1) to (5) in FIG. 12.
(1) Three laser beams L1 to L3 remain off and approach a BD sensor 209.
(2) L1 lighting signal turns to HIGH. Only the laser beam L1 lights.
(3) The laser beam L1 irradiates the BD sensor 209 and a BD signal turns to HIGH. When the BD signal turns to HIGH, the laser beam L1 turns off (L1 lighting signal=Low). The BD signal also turns to Low. At the same time, the laser beam L3 lights (L3 lighting signal=High).
(4) The laser beam L3 which lit enters the BD sensor 209. Then, the BD signal turns to HIGH.
(5) The laser beam L3 stops irradiating the BD sensor 209. Then, the BD signal turns to Low.
Time from a first rising edge (irradiation of laser beam L1) of the BD signal (measured at the time of shipping from a factory) to a second rising edge (irradiation of laser beam L3) represents a phase difference Ta between the laser beam L1 and the laser beam L3. The phase difference Ta is represented by a count value. The count value is a value represents time counted from the irradiation of the laser beam L1 to the irradiation of the laser beam L3 using a reference clock signal. That is, the phase difference Ta is a gap between the laser beam L1 and the laser beam L3. However, the phase difference Ta, measured at the time of shipping from a factory, varies to a phase difference Tb of the BD signal (measured when temperature increased) due to the increase in temperature. Thus, the phase difference Ta is previously stored, and the difference between the stored phase difference Ta and the phase difference Tb at the time of the increase in temperature is detected. In the above example, an average value of the variation amount of the three laser beams is calculated by the following expression: Variation amount Δ=(Tb−Ta)/2. Then, the light emitting timing is so adjusted that the phase difference is corrected by the average value. Thereby the writing start position of each laser beam can be aligned.
In an application (AP) which uses the electrophotographic image forming apparatus, to deal with various types of sheets (plain sheet, fine sheet), several processing speeds are often prepared for image formation. If the processing speed is changed by the AP, scanning speed of the optical scanning apparatus on the image forming apparatus side accordingly changes. The variation of the phase difference between the laser beams (which represents the gap between the laser beams) is caused due to the increase in temperature. The variation is also caused when the scanning speed is changed. On the other hand, in the conventional image forming apparatus of this kind, changing the scanning speed is not taken into account. That is, it is not possible to flexibly cope with the change of the scanning speed, which is a problem. Thus, the optical scanning apparatus which can correctly measure the gap between the laser beams even when the scanning speed is changed and when the variation is caused, rightly corrects the variation.